Gas turbine engines suitable for use in aircraft type applications can presently be placed into one of three broad categories, namely turbojet, turbofan and variable cycle engines. Turbojet engines typically comprise a rotatable compressor having a plurality of compressor blades, a combustor, and a rotatable high pressure turbine which is connected to the compressor by a shaft. In operation, the rotating compressor blades raise the temperature and pressure of air entering the turbojet engine. Fuel is mixed and burned with the air in the combustor. Some of the energy of the rapidly expanding gases exiting the combustor is converted by the turbine into rotation of the shaft which, in turn, rotates the compressor. The gases exit the turbojet engine through a nozzle such that the gases provide a force, or thrust, to the engine.
The net thrust, F.sub.n, of an engine is a function, in part, of the air flow, W, through the engine and the change in velocity of the air between the engine inlet and the engine exhaust, as shown by simplified equation (1) below. EQU F.sub.n =W*(Vj-Vo) (1)
Where:
W=total air mass flow rate PA1 Vj=exhaust gas velocity PA1 Vo=aircraft flight velocity (i.e., gas inlet velocity)
Thus, an increase in either exhaust gas velocity Vj and/or the air flow W through the engine increases net engine thrust F.sub.n. For a specific flight condition, the exhaust gas velocity Vj is proportional to the engine pressure ratio (i.e., the ratio between the exhaust gas pressure and the inlet gas pressure) and the air inlet temperature to the turbine. Specific thrust, F.sub.sp is commonly understood in the art as a measure of the thrust of an engine relative to the engine size, with engine size generally being proportional to the air flow through the engine. As used herein, the phrase specific thrust F.sub.sp is intended to mean the net thrust F.sub.n produced by an engine per pound air, as shown by equation (2) below. EQU F.sub.sp =F.sub.n /W=Vj-Vo (2)
Thus, equations (1) and (2) indicate that high specific thrust values F.sub.sp require high exhaust gas velocities Vj which, in turn, require a high overall engine cycle pressure ratio. However, high specific thrust F.sub.sp values also imply low propulsive efficiencies, .eta..sub.p, wherein propulsive efficiency .eta..sub.p is a measure of how much engine output appears as useful work supplied to the aircraft. As shown by equation (3) below, propulsive efficiency .eta..sub.p reduces to a function of the air inlet velocity Vo and the air exhaust velocity Vj. EQU .eta..sub.p =2/(1+Vj/Vo)
As shown by equation (3), large exhaust gas velocities Vj provide low propulsive efficiencies .eta..sub.p, yet generate high specific thrust F.sub.p. Likewise, low exhaust gas velocities Vj produce high propulsive efficiencies .eta..sub.p but provide reduced specific thrust F.sub.sp.
Relative to turbofan engines, Turbojet engines provide lower air flows W through the engine in combination with high engine exhaust gas velocities Vj which provide relatively high specific thrust values F.sub.sp, as shown graphically in FIG. 1. The high specific thrust values F.sub.sp advantageously provide rapid aircraft acceleration. However, the high exhaust gas velocity Vj of turbojet engines can create excessive turbulent and shear noise at low aircraft flight speeds, such as during aircraft takeoff. In addition, turbojet engines bum relatively more fuel per pound of thrust generated because of the low propulsive efficiencies .eta..sub.p of these engines.
The second category of aircraft gas turbine engines are known as turbofan engines. Turbofan engines have, in addition to the above-described turbojet engine structure (known as the turbofan engine core), a fan assembly upstream of the compressor which is driven by a low pressure turbine disposed downstream of the high pressure turbine. A portion of the air passing through the fan assembly enters an outer air duct while the remaining air enters the engine compressor. The engine bypass ratio refers to the ratio of air flow through the outer duct divided by the air flow though the turbofan engine core.
High bypass ratio turbofan engines accelerate a very large mass of air to relatively low exhaust gas velocities Vj. This combination, as shown by equation (2) results in relatively low values of thrust F.sub.n and specific thrust F.sub.sp, and relatively high propulsive efficiencies .eta..sub.p at low aircraft flight speeds. The high propulsive efficiencies .eta..sub.p reduce the amount of the fuel burned per pound of thrust. Further, the low exhaust gas velocity Vj reduces the turbulent and shear noise generated from interaction of the engine exhaust stream with the ambient air. As shown in FIG. 1, low bypass turbofan engines operate in between high bypass turbofan engines and turbojet engines. Examples of turbofan engines are discussed in U.S. Pat. Nos. 4,288,983 to O'Rourke, Jr.; 5,169,288 to Gliebe et al.; and 5,259,187 to Dunbar et al.
It is apparent to those skilled in the art that turbojet or low bypass turbofan engines having low air mass flow rates W and high exhaust gas velocities Vj are better suited for high aircraft speeds while high bypass turbofan engines become inefficient and are incapable of delivering sufficient specific thrust F.sub.sp at such high speeds. In contrast, high bypass turbofan engines are better suited for low aircraft speeds where adequate specific thrust F.sub.sp can be provided at a higher propulsive efficiency .eta..sub.p with reduced sideline and community jet noise because of the lower exhaust gas velocities Vj.
A third category of aircraft gas turbine engines are known as variable cycle engines. Variable cycle engines combine the operational characteristics of turbojet or low bypass turbofan engines with the operational characteristics of high bypass turbofan engines. For example, U.S. Pat. No. 4,080,785 to Koff et al. discloses a variable cycle turbofan engine having a core engine, first and second fans, and first and second fan bypass ducts. Both fans have variable pitch inlet guide vanes and variable pitch stator vanes. Koff et al. teach that by varying the pitch of the vanes in the first and second fans, the engine can operate in high bypass and low bypass modes, thereby providing a single engine which is efficient at both low and high aircraft flight speeds.
Koff et al. further teach that in operation, "high flow" running (i.e, open pitch) of the first fan coupled with "low flow" running (i.e, closed pitch) of the second fan results in a high bypass ratio, wherein a large quantity of air is drawn through the engine, of which only a small portion passes through the core and first fan duct. "Low flow" running of the first fan, coupled with the "high flow" running of the second fan, results in a low bypass ratio since the second fan can accept substantially all the air flow through the first fan. In addition, Koff et al. teach that maximum thrust can be delivered at an intermediate bypass ratio wherein both fan stages are set for maximum air flow therethrough. Koff et al. further state that the overall fan pressure ratio reaches a maximum at this operating point.
U.S. Pat. No. 5,311,736 to Lardellier discloses a variable cycle propulsion engine for supersonic aircraft which provides high specific thrust F.sub.sp at supersonic speeds with reduced exhaust gas velocity Vj at subsonic speeds for diminished noise emissions. Lardellier teaches a turbojet engine having rotatable fan blades disposed in an outer or bypass duct adjacent the compressor. The fan blades are connected to a drive shaft interconnecting the high pressure turbine and compressor. Variable pitch inlet and outlet guide vanes, also disposed within the outer duct, are located upstream and downstream of the fan blades, respectively. The variable pitch vanes can be provided with three pivotally connected parts such that the trailing edge of the inlet guide vanes is pivotable while the leading edge of the outlet guide vanes is similarly pivotable. The outer duct walls can also be provided with one or more inlets and discharge outlets which can provide additional air to the fan assembly depending upon the operational requirements of the engine.
At high aircraft flight speeds, Lardellier teaches that the trailing edge of the inlet guide vanes are pivoted in the same direction as the direction of rotation of the fan blades while the leading edge of the outlet guide vanes are pivoted in a direction opposite the direction of rotation of the fan blades. The outer duct inlets and discharge outlets are closed. In this configuration, the fan operates at a low rate of output. Alternatively, the outer duct can be closed by inflating bladders within the duct, thereby preventing air flow through the fan assembly. At low aircraft flight speeds (e.g., during aircraft takeoff and climb), the air inlets can be opened to supply additional air to the fan which operates at full output during take-off and climb. The inlet guide vanes are adjusted so that their leading edges and trailing edges are aligned along a line parallel to the rotational axis of the engine, and the flow straightener vanes are adjusted so that they adopt a configuration wherein the leading edge is slightly displaced from a line through the trailing edge parallel to the engine axis.
Although the above-described gas turbine engines may be suitable for the purposes for which they were intended, improved variable cycle gas turbine engines having additional benefits and features are desired. For example, a variable cycle engine which is capable of providing large swings in air flow and fan pressure ratio, more particularly, a variable cycle engine which is capable of providing maximum air flow at a low fan pressure ratio and a substantially higher fan pressure ratio at a substantially lower air flow rate would be desirable. A variable cycle engine capable of such operation advantageously provides low noise during take-off because of the low exhaust gas velocity Vj and rapid aircraft acceleration at altitude because of high specific thrust. Variable cycle engines having the above-described benefits and features in the absence of complex and weighty engine structures such as, multiple fan bypass ducts, air inlets, inflatable bladders, or noise suppression structures, would be particularly useful.